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Igneous RocksIgneous Rocks

Fig. 5.1

Genetic Classification of Genetic Classification of Igneous RocksIgneous Rocks

•• IntrusiveIntrusive: crystallized from slowly cooling magma intruded within the Earth’s crust; e.g. granite, gabbro

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Fig. 5.2

Genetic Classification of Genetic Classification of Igneous RocksIgneous Rocks

•• ExtrusiveExtrusive: crystallized from rapidly cooling magma extruded on the surface of the Earth as lava or erupted as pyroclasticmaterial.

Fig. 5.3

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Extrusive Igneous Rocks Extrusive Igneous Rocks Include:Include:

• rocks formed from the cooling of lavaslavas

• rocks formed by the cooling of pyroclasticpyroclastic material, i.e. fragmented pieces of magma and material erupted into the air

Table. 5.1

Composition and Classification Composition and Classification of Igneous Rocksof Igneous Rocks

• Chemistry: e.g. % SiO2

• Mineralogy: e.g.–Felsic–Intermediate–Mafic–Ultramafic

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Fig. 4.4

How do magmas form?How do magmas form?

When rocks melt (or partially melt).

Partial MeltingPartial Melting

Occurs when some of the minerals forming a rock melt at lower temperatures than other minerals within the

same rock

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Table 4-3

The The Formation of Formation of

Magma Magma ChambersChambers

Partial meltingPartial melting

Less dense magmaLess dense magma

Magma risesMagma rises

Magma pools in Magma pools in magma chambermagma chamber

Magma DifferentiationMagma Differentiation

The process by which rocks of various compositions can

arise from a uniform parent magma

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Magma DifferentiationMagma Differentiation

Occurs because different minerals crystallize at different temperatures (i.e., the opposite of partial melting)

Fractional CrystallizationFractional Crystallization

The process by which crystals forming in a cooling magma are segregated from the

remaining liquid

BowenBowen’’s Reaction Seriess Reaction Series

Experimental sequence of crystallization of minerals from a gradually cooling mafic (basaltic) magma

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BowenBowen’’s Reaction Seriess Reaction Series

Fig. 4.5

Fig. 4.5

Evidence of Fractional Evidence of Fractional Crystallization in the Palisades SillCrystallization in the Palisades Sill

Fig. 4.5

Evidence of Fractional Evidence of Fractional Crystallization in the Palisades SillCrystallization in the Palisades Sill

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Can fractional crystallization of a Can fractional crystallization of a primitive primitive basalticbasaltic (mafic) magma (mafic) magma

generate a generate a graniticgranitic (felsic ) (felsic ) magma?magma?

Yes, but not in the amounts present in the

continental crust!

Fig. 4.6

Modern Ideas of Magmatic Modern Ideas of Magmatic DifferentiationDifferentiation

1. Partial melting creates magma of specific composition

2. Cooling causes minerals to form and settle

Fig. 4.6

Modern Ideas of Magmatic Modern Ideas of Magmatic DifferentiationDifferentiation

3. Basaltic magma breaks through, causing turbulence

4. Crystals are mixed and deposited on chamber walls

5. Mixing of two magmas produces an andesitic magma

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Partial MeltingPartial Meltingand the Origin of Magmasand the Origin of Magmas

Partial melting of upper mantle: e.g. at divergent spreading centers

Partial melting of sedimentary rocks and mafic lithosphere: e.g. in subduction zones

Partial melting of continental crustal rocks

MaficMagmas

IntermediateIntermediateMagmas

FelsicFelsicMagmas

PlutonsPlutons

Large igneous bodies formed at depth in the

Earth’s crust

Types of Types of PlutonsPlutons

•• BatholithBatholith:: Massive, discordant intrusive body covering at least 100 km2

•• Stock:Stock: Massive, discordant intrusive body covering less than 100 km2

•• Dike:Dike: Tabular, discordant intrusive body •• Sill:Sill: Tabular, concordant intrusive body

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Fig. 4.9

Sill

Fig. 4.9Fig. 4.9

Fig. 4.9

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DikeDikeFig. 4.9Fig. 4.9

Fig. 4.10Fig. 4.10

Granite PegmatiteGranite Pegmatite

Where do most magmas form?Where do most magmas form?

• Divergent Plate Margins• Convergent Plate Margins• Mantle Plumes/Hot Spots

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Fig. 4.11

Plate DivergencePlate Divergencee.g. Mide.g. Mid--Atlantic RidgeAtlantic Ridge

Fig. 4.11

Island Arc Island Arc Plate Plate SubductionSubduction

e.g. Java, Indonesiae.g. Java, Indonesia

Fig. 4.11

ContinentalContinentalPlate Plate SubductionSubduction

e.g. Mt Raniere.g. Mt Ranier

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Fig. 4.11

Hot SpotHot SpotVolcanismVolcanisme.g., Hawaiie.g., HawaiiYellowstoneYellowstone

?

Fig. 4.12

Idealized Section Idealized Section of an of an OphioliteOphiolite

SuiteSuite

Generation of Magmas at Generation of Magmas at Divergent Plate MarginsDivergent Plate Margins

• Partially melted asthenosphere(Peridotite) rises at spreading centers, causing decompression melting of up to 15% of the rock to form mafic magma.

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Fig. 4.13

Fig. 4.13

Generation of Magmas at Generation of Magmas at Convergent Plate MarginsConvergent Plate Margins

• Subduction drags oceanic lithosphere (including a veneer of “wet” sediments) beneath the adjacent plate

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Generation of Magmas at Generation of Magmas at Convergent Plate MarginsConvergent Plate Margins

• The release of volatiles lowers the melting point of the adjacent mantle, causing fluid-induced melting to form a mafic magma, which becomes more intermediate in composition as it rises through the overlying crust

Fig. 4.14

xxFig. 4.14